Fabricating integrating circuits involves depositing and patterning layers of metal, dielectric, and semiconductor materials onto a substrate or wafer, such as silicon. The layers are patterned using optical lithography, which projects the image of a reticle onto the wafer to be patterned. Due to the decreasing size of semiconductor devices, defects which cause faults in the device are becoming progressively smaller.
An integrated circuit is typically fabricated from a plurality of reticles. On the order of 50 lithography steps are used in the fabrication of an advanced CMOS logic chip. Each reticle encodes the pattern of one physical layer of an integrated circuit. In the case of multi-patterning, more than one lithography step, hence more than one reticle, are used to pattern one physical layer. Generation of reticles and subsequent optical inspection of such reticles are standard steps in the production of semiconductors. A reticle itself is patterned by e-beam lithography using a vector-shaped beam tool, or in the case of less critical layers, by optical lithography using a laser scanner. Designers aided by electronic design automation (EDA) software start with the functional description of the IC, and after many steps produce a physical layout of each layer of the IC. The physical layout is represented by a database containing layers, and each layer containing a set of polygons. Further EDA processes such as optical proximity correction, mask proximity correction or biasing, and fracturing, convert the physical layout to data that controls the reticle writer. The reticle writer data is also contained in a polygon database. A reticle inspection system inspects the finished reticle for defects that may have occurred during its production.
A reticle needs to be fabricated free of defects in order to be used to fabricate defect-free wafer devices. Additionally, a reticle may become defective after use. Thus, there is a continuing need for improved reticle inspection techniques. A reticle is exposed to intense deep-ultraviolet radiation in the lithography projector. Photo-chemical reactions can deposit residues on the reticle. Such residues are called haze. Electrostatic discharge can damage a reticle if it is improperly handled. Therefore, a reticle is typically inspected at the wafer fab upon receipt, and periodically thereafter. A reticle under inspection in the wafer fab has already passed multiple inspections at the time of its fabrication. The objective of the inspection at the wafer fab is to detect defects that may have developed during the shipment or use of the reticle that was once known to be defect-free.